Evaluation of a large-scale bridge strain, temperature and crack monitoring with distributed fibre optic sensors

Royal Institute of Technology (KTH), Stockholm, Sweden; COWI A/S, Parallelvej 2, 2800 Kongens Lyngby, Denmark
01/2011; 1(1):37-46. DOI: 10.1007/s13349-011-0004-x

ABSTRACT Many structures like bridges are ageing and the necessity to measure the uncertain parameters is relevant. Crack-related parameters
can be measured with traditional techniques like crack gauges and displacement transducers. A method that can detect and localise
cracks as well as measure crack width is most favourable. Several distributed and quasi-distributed systems were introduced
to the market and tested in recent years. This paper presents a large-scale Structural Health Monitoring project based on
stimulated Brillouin scattering in optical fibres for an old bridge. The Götaälv Bridge is a continuous steel girder bridge
with concrete bridge deck. Steel girders suffer from fatigue and mediocre steel quality and some severe cracking and also
a minor structural element collapse have taken place. The system installed on the bridge measures strain profiles along the
whole length of the bridge and detects cracks that are wider than 0.5mm. Procedures like factory acceptance test, site acceptance
test, laboratory testing and field testing are presented and analysed. Innovative technology was developed, tested and applied
on the bridge. Heuristic knowledge was collected; conclusions are presented and discussed for future development.

KeywordsBridges–Field testing and monitoring–Maintenance and inspection

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    ABSTRACT: Many bridges worldwide are approaching the end of their lifespan and it is necessary to assess their health condition in order to mitigate risks, prevent disasters, and plan maintenance activities in an optimized manner. Fracture critical bridges are of particular interest since they have only little or no load path redundancy. Structural health monitoring (SHM) has recently emerged as a branch of engineering, which aim is to improve the assessment of structural condition. Distributed optical fiber sensing technology has opened new possibilities in SHM. A distributed deformation sensor (sensing cable) is sensitive at each point of its length to strain changes and cracks. Such a sensor practically monitors a one-dimensional strain field and can be installed over all the length of the monitored structural members, thereby providing with integrity monitoring, i.e. direct detection and characterization (including recognition, localization, and quantification or rating) of local strain changes generated by damage. Integrity monitoring principles are developed and presented in this article. A large scale laboratory test and a real on-site application are briefly presented.
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    ABSTRACT: In the last few years, the real-time monitoring of civil infrastructures has become an essential tool for the safety inspection, the design and planning of maintenance. In this context, the implementation of optic fiber sensors embedded in the structural elements is particularly useful to check strains and displacements and assess the structural safety level. In this paper, a methodology aimed at the control of the safety and serviceability level of a Prestressed Reinforced Concrete Viaduct located in the city of Bari (Italy) is presented. The procedure is based on information acquired by fiber optic monitoring system implemented during the construction of the bridge. The processing of the data provided by the sensors at different times of the execution has allowed the appraisal of the strain variations related to the load increments and to the stress losses in the different phases and the comparison with the theoretical values. This comparison enables a double check: control in the construction phases, safety in the service life.
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    ABSTRACT: Development of a model for the analysis of strain transfer mechanism in Brillouin based sensors with strain singularities is provided in this study. The main objective of the research pertained to the development of a method for accurate detection of cracks and their locations in sensing with Brillouin based fiber optic distributed sensors. The work involved formulation of a shear lag based model considering the elastic as well as elasto-plastic stages of the fiber optic coating strains. Feasibility of the proposed approach is evaluated through an experimental program. The experimental program involved use of a Brillouin Optical Time Domain Reflectometer (BOTDR) for distributed measurement of strain and detection of simulated cracks in a fifteen-meter long beam. The results indicate that the discontinuities in the strain distribution based on the theoretical analysis provide the means to accurately pinpoint the location of simulated cracks. On the other hand, the distortion effect of the BOTDR system due to averaging of the strains over the spatial resolution of the system masked the influence of strain discontinuities. In contrast to the strains acquired through the averaging process in BOTDR, the theoretical computations pertain to the actual distribution of strain along the length of the beam. Opto-mechanical relationships were also employed in order to simulate the effect of spatial resolution on the theoretical results. The apparent strains obtained in this way were compared with the BOTDR measured values. Because of the measurement noise, the apparent strains based on the theoretical computations provided better information about the location of the cracks.
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